In order to observe the morphology of a sample, such as a section of biological tissue, and simultaneously measure the distribution of the molecules existing in a predetermined area on the sample, a type of system called a mass microscope or an imaging mass spectrometer has been developed (for example, refer to Non-Patent Documents 1 and 2). These systems are capable of acquiring a distribution image (or mapping image) of the ions having a specific mass-to-charge ratio (m/z) included in any area specified on the sample based on a microscopic observation, while almost completely maintaining the original morphology of the sample without grinding or crushing the sample. Such systems are expected to be used, for example, to obtain distribution information of the proteins included in a living cell. Particularly, in the fields of medical care and pharmaceutical chemistry, those systems are expected to be used for determining a substance that specifically appears in a special type of cell, such as a cancer cell, to comprehend the distribution of the lesion.
Imaging mass spectrometers are capable of producing an optical microscope image on a sample and a mass-analysis result image at an arbitrary mass-to-charge ratio. The spatial resolution of the mass-analysis result image is normally much lower than that of the optical microscope image. For example, according to Patent Document 1, the spatial resolution of the optical microscope image is 0.5 μm, whereas that of the mass-analysis result image is as low as approximately 30 μm. Due to such a difference in the resolving power, for example, it often occurs that a plurality of kinds of tissue which have different colors or patterns and therefore can be clearly distinguished from each other on the optical microscope image cannot be distinguished on the mass-analysis result image. Accordingly, the optical microscope image can provide useful information for evaluating information obtained from the mass-analysis result image, such as the result of a statistical analysis of the mass-analysis result image, and for making judgment on that result.
An analyzing technique using the combination of the result of an imaging mass-analysis and an optical microscope image has been conventionally known from Patent Document 1. In the method disclosed in this document, after a mass spectrum is obtained for a specific micro area on a sample (e.g. a section of biological tissue), the difference between the obtained mass spectrum and a reference mass spectrum stored in a database is calculated to create a differential spectrum, and this differential spectrum is shown on a display screen. On the displayed differential spectrum, the peak of a substance that exists only in the aforementioned specific micro area (i.e. the peak not found in the reference mass spectrum) is shown by a solid line, and conversely, the peak of a substance that does not exist in the specific micro area but in the reference mass spectrum is shown by a dotted line. By performing a statistical analysis on the information of this differential spectrum, the micro area from which this spectrum has originated is classified into one of the classes (one group that can be regarded as the same kind of tissue). This classifying process is similarly performed for each micro area on the sample. As a result, the areas on the sample are classified according to their classes. However, as already noted, the size of each micro area for which the mass-analysis imaging is performed is not always sufficiently small. To address this problem, the analysis operator refers to a high-resolution optical microscope image to check the result of the classification of the micro areas by the aforementioned process, whereby the evaluation can be made with high reliability.
In the previously described conventional technique, although the result of the imaging mass analysis and the information presented by the optical microscope image are combined, this combination is actually nothing more than to simply compare the two kinds of images to evaluate the reliability of the result obtained by the imaging mass analysis. This is far from fully utilizing the two kinds of information. It is expected that a more active utilization of the optical microscope image, or more specifically, the use of an optical microscope image for the extraction or selection of the result of a mass-analysis imaging, will provide more useful and valuable information for analysis operators. However, no such technique has yet been proposed.
On the other hand, if different kinds of tissue cannot be distinguished by color, pattern or other visual information on the optical microscope image, i.e. if such visual information cannot be clearly recognized on the optical microscope image, the previously described conventional technique cannot provide useful information for the interpretation or evaluation of the result of the mass-analysis imaging. Accordingly, it is also important to develop a data processing technique for properly classifying the areas on a sample and visualizing the classification from only the result of mass-analysis imaging, without relying on the information obtained from the optical microscope image.